You are using Lync for phones but it is failing to send calls to voicemail correctly
Verify SSL certs are trusted.
- Check the event log on your Lync server (Event Viewer > Application and Services Logs > Lync Server) for event ID 14428 “TLS outgoing connection failures.”
- Check the certificate on your Exchange Unified Messaging server; if you have more than one certificate installed, you want the one tagged with the UM service
- Verify that the correct certificate for the UM services is installed and tagged for UM
- Verify that all the names the server is access by are included in the SAN portion of the certificate
- Verify that the certificate’s CA is trusted on both the UM and Lync server